Technical Field
This invention relates to certain polyoxymethylene compositions containing an encapsulated nucleant which, when extruded into stock shapes, such as rod, slab, sheet, and tube, have improved center line porosity values in comparison to comparable compositions that lack the encapsulated nucleant and that are extruded at the same rate.
Compositions based on polyoxymethylenes of relatively high number average molecular weight (i.e., 10,000 to 100,000) are useful in preparing semi-finished and finished articles by stock shape extrusion, compression molding, injection molding, blow molding, rotational molding, melt spinning, stamping and thermoforming techniques commonly used with thermoplastic materials. Finished articles made from such compositions possess desirable physical properties, including high stiffness, strength, low coefficient of friction, and good wear and solvent resistance. However, in certain applications, especially stock shape extrusion, it is desirable to reduce the center line porosity of the polyoxymethylene composition. By the present invention, the center line porosity of polyoxymethylene compositions is found to be lowered, in comparison to comparable compositions, extruded at the same rate by the inclusion of therein of low levels of particular types of encapsulated nucleants.
The compositions of the present invention are especially useful in those applications where a polyoxymethylene composition is extruded into a stock shape and where it is desired that such stock shapes have reduced center line porosity, without slowing the rate of extrusion. Stock shapes include rods, slabs, sheets, tubes, and other symmetrical shapes. Rod and slab stock shapes are used as blanks in machining processes. Sheets are used for die cut parts (such as gears, washers, and gaskets). Tubes are used for control cable inners (which are small diameter tubes that serve as conduits for push/pull cables).